QUESTION:
Even though Mars is very dry is it still possible that Mars 
has tornadoes?

ANSWER from Jim Murphy on February 15, 1997:
Yes, tornadoes might be possible on Mars, even though Mars is so dry.

What exactly leads to the development of tornadoes here on Earth is not
very well understood, but the main ingredients are:  hot, moist air near the
ground and cooler or cold drier air above;  wind speed and direction which 
change with increasing distance above the ground such as winds from the south
near the surface changing to faster winds from the west (in the northern 
hemisphere) a few kilometers above the surface.   These changing wind speeds
and directions help form a rotation so that air which rises gets turned.  The 
water vapor which condenses to form rain in the rising air adds heat to the
air, and causes the air to rise further and helps create the general unstable 
atmosphere (much like a beach ball taken to the bottom of a pool wants to 
rise up to the water surface due to air being less dense than the water).

As you already know, Mars does not have much water vapor available to help
the process.   But, Mars probably does have situations where the wind
direction changes with height in a favorable way, and Mars can have 
the situation where the air near the ground is very warm and the air above
it colder.  This can be especially true if there is some dust in the air
near the ground, since the dust absorbs the sunlight and gets warm, and then
warms the air near it (the dust).

We have not seen any features on Mars (from pictures on spacecraft orbiting
Mars or from the Viking landers) which we can say really are tornadoes.
Some scientists think that some long, straight, dark features on the
surface of Mars, and visible in spacecraft pictures, might be signs that
a tornado passed by there. By lifting dust from the ground over which
the tornado passed, the tornado might have left evidence of its passage.
I personally am not convinced that this is good evidence for tornadoes 
occurring on Mars.   I hope that our current (Mars Pathfinder and Mars 
Global Surveyor) spacecraft missions to Mars will help us learn if in 
fact Mars does have tornadoes.

Some Viking orbiter pictures show tall (maybe a few miles!) columns of dust.
These columns are about 100 meters wide at the surface.    These are thought
to be 'dust devils', and not tornadoes.  On Earth, dust devils are generally
smaller and less intense than tornadoes.  While tornadoes require an unstable
atmosphere, air heating due to condensation, and changes in wind direction and
increasing wind speed with height, dust devils require a very hot surface (and
air in contact with it) and little variation in the wind with height.   These
dust devil conditions are common over deserts in the summer.  The 'dust devils'
(if that is really what they were) seen on Mars were all seen during summer,
when conditions were more like what we expect for dust devil formation than
for tornado formation.